1 Idaho potato, peeled and quartered
1 stick unsalted butter, softened, plus more for greasing
1 cup granulated sugar
1 large egg
¾ cup whole milk
1 teaspoon vanilla
1 cup all-purpose flour
1 teaspoon baking powder
2 firm but ripe pears (D’Anjou), cored and thinly sliced
1/3 cup caramel sauce
In a medium saucepan, add the potato and enough cold water to cover. Bring to a boil and cook until potato is fork tender, 8 to 10 minutes. Drain and mash.
Heat oven to 375 degrees F and grease a 9-inch round cake pan. In the bowl of a standing mixer (or using a handheld mixer), beat the softened butter and sugar until fluffy, about 3 minutes.
Scrape down bowl with a spatula and beat in the egg. In a separate bowl, whisk the milk and vanilla into the mashed potatoes then beat into the batter.
Add flour and baking powder to the mixing bowl and beat until the batter is smooth, about 2 minutes.
Spread out the pear slices along the bottom of the prepared cake pan, making an even layer of fruit. (No need arrange slices in any particular way.)
Drizzle the caramel sauce over the pears (if the caramel sauce isn’t pourable, warm it before drizzling). Pour cake batter into the pan and bake until sponge-like, 45 to 50 minutes.
Place on a cooling rack. When cooled, invert onto a serving platter.